2.1 Phytate
Phosphorous is an essential dietary requirement for animal growth.
Abundant stores of phosphorous are present in plant seeds in the form of phytate (myo-inositol hexakisphosphate). Phytate serves as the storage form of phosphate and myo-inositol for germination and seedling growth. Phytate is synthesized during seed development and comprises 1-3% of the total dry weight of seeds (Reddy et al., Phytates in Cereals and Legumes, CRC Press, Inc., Boca Raton, Fla., 1989). In cereals, phytate is deposited in aleurone particles. It is found in protein bodies in legume cotyledons. A biochemical and ultrastructural analysis of soybean seeds showed the presence of phytate in protein bodies, with a significant amount deposited with globoid inclusions (Prattley and Stanley, J. Food Biochem. 6:243-253. 1982).
The soybean [Glycine max (L.)] has long been recognized as a valuable component of medicine, food and animal feed. Soybean is an important crop worldwide as both a protein and oil source. Soybean is processed to yield soybean oil and soybean meal by a hexane extraction process. Because of its high nutritional value, wide availability, low cost and consistency of composition, soybean meal has displaced most of the animal proteins (meat meal, tankage, and fish meal) and has become a vital component of poultry and swine feeds. Together, poultry and swine feeds account for over two thirds of soybean meal utilization in the United States. Phytate accumulates in soybean during seed development, where it serves a phosphorous storage function. Phytate is normally utilized after germination of the soybean seedling, when it is broken down by newly-synthesized phytase.
Phytate is poorly utilized by humans and other monogastric animals. Because non-ruminant animals lack the enzymes necessary for phytate degradation, phytate-bound phosphorous passes through the gastrointestinal tract undigested, making it unavailable to non-ruminants. Phytate is considered an anti-nutritional factor because it chelates essential minerals such as zinc, manganese, iron, calcium and potassium and forms indigestible protein-phytate complexes (Erdman and Forbes, J. Am. Oil Chem. Soc. 58:489-495 (1981); Zhu et al., Anim. Feed Sci. Techn. 27:341-351 (1990); Simons et al., Brit. J. Nutr. 64:525-540 (1990)). Plant phytate is present predominantly as the calcium-magnesium-potassium salt called phytin. These complex forming properties of phytate are responsible for lowered mineral and protein bioavailability in animal diets containing high phytate levels (Reddy et al., supra).
To meet dietary requirements, inorganic phosphorous is routinely added to swine and poultry rations. Phosphorous supplementation increases feed costs and contributes to the phosphorous content of animal waste. Moreover, non-utilized phytate is excreted by the animals contributes to environmental pollution in areas of intensive animal production. Annually in the United States, swine and poultry generate over 20 million tons of manure containing over 300,000 tons of waste phosphorous (Cromwell and Coffey, Altech Conference Proceedings, 1991). Typically, manure is applied as fertilizer to pastures and croplands, causing an increase in soil phosphorous, eventually resulting in water contamination. Since phosphorous is the limiting nutrient for aquatic plant life growth, increases in water phosphorous content results in eutrophication and decreased water quality. Increased regulatory scrutiny of animal waste disposal has fostered interest in finding solution for decreasing phosphorous output.